High pressure sodium arc lamps have been in commercial production for many years and have been subject to many improvements in design, materials and processing. Such lamps include a translucent ceramic tube, an outer envelope including an electrical connector, and a frame for supporting the arc tube within the outer envelope. The frame is electrically conductive and carries power to the arc tube. The arc tube is typically fabricated from polycrystalline alumina or yttria and contains an amalgam of mercury and sodium for producing light having a desired output spectrum. Tungsten or molybdenum electrodes are positioned within the arc tube at opposite ends and are attached to feedthroughs selected to have thermal expansion characteristics closely matched to those of the ceramic tube. The feedthroughs are hermetically sealed in openings at opposite ends of the arc tube. Niobium, usually containing one percent zirconium by weight, is the Preferred feedthrough material for alumina arc tubes.
A variety of electrode feedthrough structures and techniques for hermetically sealing the feedthrough to the arc tube are known in the art. In one widely-used structure, the feedthrough is a niobium tube and the tungsten electrode is welded to one end of the tube. The opening in the end of the arc tube is sufficiently large for insertion of the electrode, and a tube is used to reduce stresses due to thermal expansion mismatches and to limit material costs. Typically, a ceramic ring is sealed to the end of the arc tube over the feedthrough to extend the length of the seal and to improve its reliability. Such a structure is disclosed in U.S. Pat. No. 4,539,511. The niobium tube with the electrode attached is sealed in the opening in the end of the arc tube with a suitable sealing frit. A region of the niobium tube close to the arc tube becomes wetted with the frit material. The end of the niobium feedthrough outside the arc tube is welded to the lamp frame or to a wire that is attached to the lamp frame. In the prior art, the welding of the feedthrough to the lamp frame often produced cracking of the seal between the feedthrough and the arc tube unless the niobium tube was extended in length so that the welded connection between the tube and the lamp frame was spaced from the hermetic seal. While this structure provides satisfactory performance, it is relatively expensive because of the material used in the extended length of niobium tube.
High pressure discharge lamps utilizing tubular electrode feedthroughs are disclosed in U.S. Pat. Nos. 4,545,799 to Rhodes et al; 4,539,511 to Denbigh et al; 4,019,078 to Burgess; and 4,501,799 to Driessen et al. A discharge lamp utilizing a solid rod for an electrode feedthrough is disclosed in U.S. Pat. No. 4,101,799 to Wiedijk et al. Naturally, more material is utilized in a solid rod, thereby increasing the material cost.
Another feedthrough structure known in the prior art utilizes a niobium wire sealed in an opening in the arc tube and connected to the tungsten electrode. The niobium wire is smaller in diameter than the electrode and the electrode will not pass through the opening in the end of the arc tube. In this case, the niobium wire and electrode must be sealed into a ceramic end plug which is then sealed to the end of the arc tube, as shown in U.S. Pat. No. 4,199,704 to Varshneya et al. This arrangement requires a seal not only between the niobium wire and the end plug, but also between the end plug and the ceramic tube. The seal between the end plug and the arc tube is extremely difficult to make and has inherent problems. Stresses occurring in this design create cracks in the seal area and subsequent lamp defects.
It is desirable to provide a ceramic arc tube assembly having an electrode feedthrough structure with reduced cost relative to prior art structures, while providing a reliable, long-life seal.
It is a general object of the present invention to provide improved ceramic arc tube assemblies for high pressure discharge lamps.
It is another object of the present invention to provide an electrode feedthrough structure for a ceramic arc tube assembly wherein the quantity of material used in the feedthrough is reduced.
It is still another object of the present invention to provide a ceramic arc tube assembly which is low in cost and easy to manufacture.
It is a further object of the present invention to provide a ceramic arc tube assembly that is easily attached to a lamp frame.
It is still another object of the present invention to provide a ceramic arc tube assembly having a long operating life.